This invention relates generally to communications systems, and more particularly to adaptive canceller architectures for use in multiple carrier frequency reuse communication systems, such as satellite communication systems.
In a conventional duplex communications system in which each of the sites of the system transmits and receives information to and from the other site, each site transmits on one frequency and receives information on another frequency. As a result, duplex communications require twice the bandwidth of a unidirectional communication link. This is costly where the available bandwidth is limited since it restricts the number of sites which can communicate over the system. This is particularly true with satellite communications systems. With satellites, transponder bandwidth is limited and system resources are costly.
In many relay-based communications systems, such as satellite links where the two transmitter/receiver sites will be in the same satellite beam, a downlink will be seen not only at the intended receiver site but also at the transmitter source of the downlink signal. Thus, each site receives not only the intended downlink signal from the other site, it also receives a replica of its own transmitted uplink signal. The replica comprises the uplink signal delayed in time by the roundtrip transmission time between the transmitter and the satellite, and it will be at a different signal level of course. Satellite and similar relay systems do not generally demodulate a received uplink signal, but merely translate, i.e., shift, the frequency of the uplink signal to a different frequency and transmit it back to the ground. The satellite on-board electronics that receives the uplink signal, frequency translates it to the downlink frequency, and transmits the downlink signal back to earth is called a transponder. Thus, the satellite transponder may receive uplink frequencies from two different sites in, e.g., the 6 GHz range, and translate the received signals to downlink frequencies in, e.g., the 4 GHz range. Each site could be assigned a different uplink frequency in the 6 GHz range and a different receive frequency in the 4 GHz range. As stated earlier, this requires twice the bandwidth of a unidirectional link.
Where both sites and, therefore, both the uplink and downlink signals are in the same satellite spot beam, systems have been developed that make more efficient use of bandwidth. These systems allow reuse of frequencies by two sites, thereby eliminating one uplink frequency and one downlink frequency. This doubles the available bandwidth of the satellite transponder. The basic idea behind these systems is that the received downlink signal at the first site is a composite signal comprising the sum of the desired signal of interest from the second site combined with a replica of the first site's own transmitted uplink signal. Thus, the signal of interest can be obtained by subtracting from the composite downlink signal the portion of the downlink signal due to the first site's own uplink signal. This requires generating a local replica of the uplink signal that has been relayed by the transponder. The relayed portion or replica of the uplink signal from the first site, received back on the downlink, is treated as interference and is removed from the composite downlink signal by an interference canceller.
In these known systems, the interference canceller is merely a device which produces and subtracts the replica of the interfering signal from the composite signal to give the signal of interest. The replica is derived from the uplink signal by estimating certain parametric changes imposed on the relayed uplink signal due to its roundtrip through the satellite transponder. These include not only the roundtrip time delay, but also the changes in signal level, frequency and phase that are experienced by the relayed uplink signal. These changes or parameters are estimated by the interference canceller and are applied to a local sample of the uplink signal as it is transmitted to produce the replica which is subtracted from the composite signal by the interference canceller.
The degree of cancellation of the interfering signal depends upon the accuracy with which the compensation parameters applied to the sample of the uplink signal can be estimated. Unfortunately, the changes experienced by the uplink signal are not static. Rather, they are dynamic and vary in time. Variations result from different causes. For one, satellites are not stationary, but tend to precess about a nominal location or position. This causes the distance between a transmitter/receiver site and the satellite to vary with time, which produces a varying time delay. It also produces variations in frequency and in phase due to Doppler shifts, and produces signal level variations. Moreover, the frequency translation circuits in the satellite transponder may also drift or vary in time, producing additional variations which are imposed upon the interfering downlink signal. Other effects that are relatively static but unknown include frequency-dependent amplitude and group delay distortion due to electronics at both the transmit and receive sites as well as in the satellite. The interference canceller must mimic these distortion components to provide adequate cancellation. Known interference cancellers attempt to estimate and track the static and dynamic variations in parameters to improve the accuracy of cancellation. However, this is a complex task. Known interference cancellers tend to be rather complicated, and lack the desired precision and accuracy in estimating the cancellation parameters to afford desired levels of cancellation of the interfering signal.
Additionally, known interference cancellers must be specifically designed or adapted for the satellite receiving systems with which they are intended to be used. This is because the interference canceller must be specially designed to interface to and be compatible with the satellite receiving system. This complicates the task of retrofitting or upgrading existing satellite receiving systems, or providing a universal interference canceller, since the interference canceller must be designed to be compatible with the electrical and mechanical specifications of the system hardware and software. Known interference cancellers also frequently must exploit a priori knowledge of certain external signal characteristics or the underlying data stream used to generate the uplink signal. This dependence also complicates any efforts to adapt the canceller to other systems.
It is desirable to provide adaptive interference cancellers which solve these and other problems of known interference cancellers by improving the compensation parameter estimation and cancellation functions while reducing complexity in the cancellers, affording a higher degree of precision and accuracy in canceling interfering signals (hence providing better performance), and leading to cost-reduced solutions. It is to these ends that the present invention is directed.